This invention relates to an illumination or erase bar for a xerographic printer or the like, having a row of light emitting diodes for illuminating an image plane in a stripe having approximately uniform intensity along the length of the stripe.
There are times when it is desirable to illuminate a selected area on a photosensitive surface in a xerographic device such as a printer, copier, scanner, facsimile machine, or the like. For example, the entire breadth of the medium may be illuminated as it moves along after transferring an image to paper, for completely discharging the medium so that it is in a uniform ground state to be charged for receiving a new image. The entire breadth of the medium may be bathed in light of reasonably uniform intensity for this total erasure.
There are other times when it is desirable to illuminate a selected area on the medium. This might be done, for example, to block out text or, in a color printer, to paint in background color. It might be used, for example, in a color printer, to block out the area of a letterhead on a document so that the letterhead is printed in color and the text of the letter is printed in black. A more sophisticated erase bar is needed for such selective illumination.
Such an illumination bar may illuminate two or three millimeter long segments or pixels of a stripe a two or three millimeters wide. Each of these pixels is illuminated by a separate light emitting diode (LED). Each of the LEDs can be separately addressed or switched for illuminating a desired area on the medium. It is still important to obtain reasonably uniform illumination within the area being illuminated; typically, an intensity difference of about two to one between the brightest and dimmest areas is acceptable. Sometimes tighter specifications for differences in illumination are required. This means that the illumination within each pixel must be reasonably uniform. It also means that there cannot be gaps between adjacent pixels.
Typically, one seeks to achieve an intensity of illumination across a pixel in the direction of the length of the row or stripe which is approximately trapezoidal. Over the major portion of the length of the pixel, the intensity is reasonably uniform. At each edge, intensity drops off rapidly with distance toward a dark area surrounding the pixel. It is undesirable to have a halo of light surrounding the illuminated pixel. Adjacent pixels in the row are arranged so that the decreasing illumination of one pixel overlaps the increasing illumination of the next pixel, preferably with the overlap making the two 50% intensity levels of adjacent pixels at the same location. This avoids any gap in illumination, any significant decrease in illumination between adjacent pixels, and any significant increase in illumination due to excessive overlap.
Bubble lenses and the like have been used for projecting the light from an LED toward an image plane for providing individually illuminated pixels. Lens systems are, however, costly and somewhat inefficient in transmitting light from the LED to the image plane. In a typical system, only about 20% of the light from the LED may be usefully projected on the image plane. Defects in an LED or misalignment of an LED may also be projected toward the image plane, thereby degrading the uniformity of light within a pixel. For example, a chipped corner on an LED may show up in the image plane as a significant change in illumination. Another difficulty with lens systems is a limited depth of field. A belt carrying the photosensitive medium may "flap" somewhat, varying the distance between the erase bar and the medium. With a lens system this can change the shape and size of the illuminated pixel as the medium moves in and out of focus.
It is, therefore, desirable to provide an individually addressable LED illuminator bar which is less costly and more efficient than a lens system. It is desirable that the system be tolerant of LED defects. It is desirable that the intensity of illumination be approximately uniform across each pixel and between pixels, so that a uniform intensity illumination is obtained across the full length of the illuminator bar, across any subset of pixels, and across each individual pixel.
An exemplary illuminator bar and some prior techniques for illumination are described and illustrated in U.S. Pat. No. 4,759,603 by Jones. In the arrangement described therein, there is appreciable absorption of light. It would be desirable to provide an LED illuminator bar with higher efficiency.